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Source: Student Loan Forgiveness Support via Facebook.

Starting to invest early in your life is one of the best ways to become financially secure, and one of the Motley Fool's core missions is to help educate would-be investors on the basics of how to get their money working harder for them. In order to help investors get started, we got seven of our Motley Fool contributors to share some advice based on their years of investing experience. See how their guidance can help you in your investing.

Todd Campbell: Kicking off a career means earning far more money than ever before, yet most young Americans fail to take advantage of the benefits of putting their new found wealth to work right away. Don't be one of them.

According to the Federal Reserve, nearly half of all Americans under the age of 30 have yet to set aside a dime for retirement and that means that millions of Americans are missing out on the benefit of compounding interest, which is interest that can be earned on interest that has been earned in previous periods. 

Invest young enough and the interest-on-interest phenomenon can be worth hundreds of thousands of dollars in your golden years. Consider this example. John invests $300 per month from age 40 to age 65 and earns a hypothetical 6.5% on this money every year. At age 65, John would have a nest egg of $212,000. That's not bad, but if John starts at 25 instead of 40, that nest egg would grow to $632,287, or nearly three times as much!

Matt Frankel: If there's one piece of investment advice I wish I had known from the beginning, it's this – trying to time the market is almost always a bad idea. Too many investors try to catch stocks at the bottom, only to buy and see their investments plunge even further. Or, they stop buying stocks altogether when the market reaches a certain level, only to miss out on the next leg up.

So, when is it a good time to buy stocks? Always.

Through an investment principle known as dollar-cost averaging, you can put the odds in your favor. Basically, by investing set dollar amounts over time, you end up buying more shares when prices are low and less when prices are high.

For example, let's say that you like a certain stock and it trades for $40 today. So, you decide to invest $1,000, which buys 25 shares. And, six months later the stock's price rises to $50, so investing another $1,000 will get you another 20 shares – bringing your total to 45 shares.

So, the average price of this stock (over the two times you bought) was $45. However, since dollar-cost averaging allowed you to buy more shares when the price was low, your average cost per share is $44.44. Of course, you would have been better off buying all of your shares at $40, but you had no way of knowing what the next move would be.

Dollar-cost averaging allows you to accumulate stocks at favorable average prices, while eliminating the fool's errand of trying to time the market.

John Maxfield: If I could share only one piece of advice with young investors, it would be this: Don't over-complicate things.

One way to invest is to closely follow individual stocks and then try to discern the best times to buy and sell them.

Unfortunately, however, the data proves that this approach generally leads to returns that trail the broader market. The problem is that humans are emotionally designed to buy when everyone else is buying (when prices are high) and to sell when everyone else is selling (when prices are low).

An alternative approach, in turn, is to simply invest on a pre-determined schedule into a low-cost exchange-traded fund that tracks the S&P 500 -- say $500 on the first day of trading each month. This is essentially what Warren Buffett encourages individual investors to do, and it's the approach that people on Wall Street tend to use for their own money.

If this sounds too simple, it's worth keeping in mind that the S&P 500 tracks the biggest and best publicly traded corporations in America. While some of these businesses will undoubtedly come and go as time progresses, the group as a whole should continue to grow, make money, and reward their shareholders for longer than any of us will be around.

Sean Williams: If I could hop in Doc Brown's DeLorean a la Back to the Future and talk to myself two decades ago, I'd like to browbeat the idea into my head that you're going to lose money on some of your investments, and you can't always be right.

One of the biggest problems with investing is that emotions and ego tend to get in the way. I'm a very competitive person and I like to win – at everything. But, the thing about investing is that you're only competing against yourself.

Think about Warren Buffett, arguably the most renowned investor of our time. Buffett has a long history of outperforming the broader market averages, but even he's made some awful investments. Recently Buffett sold his stake in U.K.-based Tesco for a substantial $444 million loss. Buffett even admitted that he made a "huge mistake" by letting his losses grow despite an accounting scandal and multiple earnings warnings from Tesco.

The point here is simple: be willing to cut your losses on stocks that no longer align with your investing thesis and hang onto your winners for as long as possible since time and compounding are your greatest investing allies.

Selena Maranjian: Any investor just starting out would be well served by learning a lot about investing -- and to keep learning, for the rest of his or her investing life. Read a lot. The Motley Fool has a host of helpful books, such as, The Motley Fool Investment Guide for Teens. Peter Lynch's classic Learn to Earn is an insightful introduction, too.

Read the classics, such as Lynch's One Up on Wall Street, Philip Fisher's Common Sense and Uncommon Profits, and Benjamin Graham's The Intelligent Investor. Read biographies, such as Buffett: The Making of an American Capitalist by Roger Lowenstein, and profiles of other great investors, such as John Train's Money Masters of our Time. Don't just focus on investing -- read about great businesses, too, as great investors need to be able to spot great businesses.

Talk about investing with your friends and relatives. Learn from them and perhaps form an investment club to share research and ideas. Great investors never stop learning.

Dan Caplinger: The piece of advice I'd give new investors both young and old is not to be in a hurry to take profits on your winning stocks. If you make smart choices with the goal of investing for the long run, you'll inevitably face a situation in which the stock you picked explodes higher in value, and it'll be tempting to lock in those gains rather than running the risk of seeing your paper profits evaporate.

The key, though, is understanding the true nature of a long-term investment. When you have a time horizon that extends for decades, you can expect the power of compounding to produce total returns that will increase your initial investment tenfold. Extraordinary growth stocks can sometimes give you returns of 10,000% or more over a long period of time. When you consider it from that perspective, selling just because the stock you chose doubled seems incredibly short-sighted.

That doesn't mean you should never sell. If you have a prospect that's even better than the stock you own, then trimming a winning position to provide cash for investing in a new position can be a smart move. Moreover, fundamental changes in a company can sometimes hurt its long-term prospects, going against the reason you bought the stock in the first place and justifying a possible sale. Still, you shouldn't be in a hurry to sell just because you have a profit. Doing so can end up costing you far more than you gain.

Jordan Wathen: If I could go back in time and give myself one helpful hint, it would be to invest in what you know. Looking back, my losers are concentrated in particularly difficult to understand industries like energy. My biggest winners were in things that made sense to me; businesses like insurance, consumer goods, and some very simple industrial companies.

I think it's safe to say that if you're not interested in everything you read about a particular industry, you probably shouldn't go off on your own and invest in it. If keeping up with your investments isn't fun, then you're already at a disadvantage to other investors who take pleasure in watching a company's every move.

Speaking broadly, there are 10 market sectors, and more than 3,500 publicly traded stocks. Actively invest in the ones you find enjoyable, and let a fund manager or index fund pick up the industries you don't particularly like. I can't guarantee it'll net you a better return on your money, but it will certainly net you a better return on your time. And you'll be happier for it.
 

Neither the participants in this roundtable nor The Motley Fool has any position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.